Quoting from the official VMware announcement email:
As part of our strategy to make server virtualization universally accessible, VMware is introducing VMware Server – a free virtualization product for Windows and Linux servers.
VMware Server is the next generation of VMware hosted server virtualization offering
VMware Server is VMware’s next generation hosted server virtualization product with support for two-way Virtual SMP, 64-bit guest operating systems and Intel Virtualization Technology. VMware Server is an unlimited CPU license and is available for immediate download as a beta version.
VMware Server is an upgrade to GSX 3.2 and replaces GSX Server on the market
The free VMware Server represents the upgrade path for all GSX Server customers. Once VMware Server is generally available, which is currently planned for Q2 2006, it will replace GSX Server as VMware’s hosted server virtualization offering and VMware will no longer market and sell VMware GSX Server. The free VMware Server represents the upgrade path for GSX Server customers.
GSX Server will be fully supported by VMware for two years
VMware is fully committed to GSX Server customers’ continued success. Accordingly, notwithstanding our Support and Subscription agreement terms, GSX Server will be fully supported by VMware for 24 months (two years) after VMware Server becomes generally available and replaces GSX Server.
When VMware Server becomes generally available, VMware will introduce paid Support and Subscription services for VMware Server. Since VMware Server replaces GSX Server on the market, GSX Server customers with valid Support and Subscription contracts will receive technical support for VMware Server as part of their existing contracts.
GSX Server customers can upgrade to the free VMware Server or to VMware Virtual Infrastructure
GSX Server customers can continue to expand their deployments by deploying the free VMware Server in addition to GSX Server, or by migrating the entire GSX Server deployment to VMware Server.
VMware also offers the option of upgrading from GSX Server to VMware Virtual Infrastructure products ESX Server with Virtual SMP and VirtualCenter with VMotion.
Read more at the GSX Server FAQs page.